Since the 1986 paper of P. Powell-Abel et al [see Science 223:738] showing that plants transformed with and expressing the coat protein gene of tobacco mosaic virus (TMV) are resistant to TMV, there have been a nunnber of other examples of this concept which will undoubtedly have important implications for the protection of many crop species from various viral infections. To date, for example, viral coat protein-mediated resistance has been shown with at least 25 viruses in 15 taxonomic groups including alfalfa mosaic virus, tobacco rattle virus, potato virus X, cucumber mosaic virus (CMV), potyviruses, and plants transformed with both potato virus X and potato virus Y coat protein.
Plant virus sequences other than those coding for the viral coat protein have been tested to determine if transformed plants can be made to exhibit resistance to post-transformation viral infection. Positive sense sequences of alfalfa mosaic virus comprising almost full length copies of RNAs 1 and 2 failed to induce resistance in transformed plants [see Virology 163:572 (1988)]; anti-sense sequences of the TMV and potato virus coat protein genes did induce a low level of resistance in transformed tobacco [see Proc. Natl. Acad. Sci., USA 86:6949 (1989); and EMBO Journal 7:1273 (1988)]; likewise antisense RNAs from one of three regions tested (5' sequences or RNA 1) of the CMV genome gave a low level of resistance in only a single transformant line.
Other forms of resistance using plant transformations with DNAs prepared from satellite RNAs of plant viruses have been reported, such as the use of the satellite of CMV [see Nature 328:799 (1987)] and the concept of the ribozyme based on sequences from satellite RNAs which possess the capacity to self cleave [see Nature 334:585 (1988)].